Separate Modifiability, Mental Modules,
and the Use of Pure and Composite Measures to Reveal Them

How can we divide a complex mental process into meaningful parts? In
this paper I explore the possibility of considering parts of a process
as especially interesting if they are modular in the sense of
being separately modifiable. Evidence for separate
modifiability is provided by an instance of selective
influence: two factors F and G (usually
experimental manipulations) such that part A is influenced by
F but invariant with respect to G, while part
B is influenced by G but invariant with respect to
F. Such evidence also indicates that the modules are
functionally distinct. If we have pure measuresMA
and MB, each of which reflects only one of the parts, we need
to show that MA is influenced by F, but not
G, while MB is influenced by G but not
F. To demonstrate separate modifiability if we have only a
composite measureMAB of the entire process usually
requires us also to test a combination rule for how the parts
contribute to MAB.

I present a taxonomy of separate-modifiability methods, discuss their
inferential logic, and describe several examples in each category. The
three categories involve measures that are derived pure (based on
different transformations of the same data; example: separation of
sensory and decision processes by signal detection theory), direct
pure (based on different data; example: selective effects of
adaptation on spatial-frequency thresholds), and composite
(examples: the multiplicative-factor method for the analysis of
response rate; the additive-factor method for the analysis of reaction
time). Six of the examples concern behavioral measures and functional
processes, while four concern brain measures and neural processes.
They have been chosen for their interest and importance; their
diversity of measures, species, and combination rules; their
illustration of different ways of thinking about data; the questions
they suggest about possibilities and limitations of the
separate-modifiability approach; and the case they make for the
fruitfulness of searching for mental modules.

NOTE: Based on a contribution to the 1998 symposium
in Amsterdam, "Looking for Stages," honoring Andries Sanders.